The Arthropoda agree in the characteristic already mentioned, the articulations of their limbs, whence the class-name is derived. The body presents various degrees of complexity. In the caterpillar, the metamera, somites, somatomes, or annuli, owe their mobility to differences in thickness of the integument. In the Myriapods the numerous similar somites are flexed on each other by the overlap of the chitin-thickened portions of cuticle which protect the upper and lower surface of each division. The somites are more or less effaced in the abdomen of insects and spiders; head and thorax in crabs and spiders have their composite origin concealed. But the external signs of division of the body no longer correspond, as in Annelids, to the distribution of the internal organs, which, with a partial exception in the case of the nervous system, are now unities contributing to the well-being of the whole. Homonomy, the absence of segmentation, or the equivalence of the divisions of the body, among the Annelids, has been contrasted with the heteronomy, or segmentation of the arthropod body. The difference, however, is only one of degree, since both the cephalic and caudal extremities, at least of the higher Annelids, are true segments, i.e., fused somites which, in addition to fusion, have undergone some amount of specialisation. In the four classes of Arthropods the head is a constant segment. It consists of præ-oral and post-oralsomites, the ganglia of which are represented by the supra- and sub-œsophageal masses. The number of somites, as represented by appendages, is not the same in the four classes, and as the variation affects the præ-oral appendages supplied from the supra-œsophagealganglion the difference is of great importance. In the Crustaceans the somites of this segment are, according to Huxley,—

The Podophthalmata present the eyes as modifications of processes identical with those which become ambulatory limbs. In the rest the eyes are sessile. In Myriapods, Arachnids, and Insects, the eyes are sessile, and the præ-oral appendages are reduced to one pair of antennæ, whose innervation is from the supra-œsophagealganglion. Apart from the value to be assigned to descent in the search for homologies among these classes, it is a question of fact whether the eyes are præ-oral or lateral to the oral aperture. The cephalic lobes carry the organs of sight probably in the earliest types of development at the angle of bifurcation, the position of the single eye of Ostracods. In more complex forms the eyes appear more or less towards the outer margin of the lobes; and in insects where the cephalic arch is high, these organs may appear to correspond not to the most anterior, but to a posterior part of the cephalic sterna, just as sensory organs appear, the gustatory at the base of the outer, the auditory at the base of the inner antennæ in the higher Crustaceans. By shortening of the development process the change of position may be obscured, and the eyes, primitively belonging to the extremity of the embryonal body, may from the first appear connected with more posterior somites. The identification of homologous parts of the præ-oral region in the four classes rests on the opinion held as to the origin of the classes. If the Crustaceans are regarded as the stock of the Arthropods, the homologies must be recognisable. If, on the other hand, all four are divergencies from a common stock, then the absolute identity of the parts must hold a second place in comparison with a general conformity to the common plan. The identification of the eyes with a particular pair of appendages necessitates the assumption that these sense organs, when sessile, are so by non-development of their supports. The converse supposition is more admissible, that the eyes are supported on stalks as the result of an adaptive modification. Further, among the Crustaceans we find hints of the primitive composite character of the Arthropod. The auditory sacs of Mysis are at the caudal extremity of the body, the respiratory organs of Isopods are in the same position, and the genital orifices vary in different genera, and even in the sexes of the same species. Analogous (perhaps no more) is the distribution among Molluscs of the eye spots which fringe the mantle of Pecten, are pedunculate in the snail, and, with the otolithic sacs, are in close proximity to the nerve centres of cuttlefishes. If this view is accepted, the close comparison of the limbs of Arthropods loses much of its importance, and it becomes more interesting to endeavour to trace the primitive form from which the divergences have occurred. Among Crustaceans the Nauplius is the earliest recognisable form,—“an unsegmented ovate body, a median frontal eye, and three pairs of natatory feet, of which the anterior are simple, and the other two biramose” (F. Müller). The third pair of appendages is replaced by the mandibles, the oval body is divided by a transverse fold, and the Naupliushead and tail thus marked off have the mid-body of the adult developed by intercalation between them. Appendages are developed before segmentation is indicated in the free living Nauplius; but in some this stage is overpassed in the egg, the evidence of its existence being the presence of a thin exuviated membrane which is not eggmembrane, nor can it be termed amnion, without overstraining that term which is properly used in the higher vertebrateembryology. In Insects the vermiform stage is rapidly passed through, the priority of segmentation to the development of appendages being indicated in the Trichoptera, according to Zaddach, and in Aphis, according to Huxley. If we go to the Rotifers, there are in that group types which are comparable with the Nauplius of Crustaceans, and with the vermiform larvæ of Insects, as O. Schmidt and Lubbock (Origin and Metamorphoses of Insects) have shown. Pedalion mira (Hudson) has a very close resemblance to the Nauplius, Lindia to the vermiform grub of Dipterous insects. The resemblance is not impaired by the comparison suggested by Ray Lankester between the Molluscs and Rotifers. Huxley calls the Molluscs “little more than oligomerous modifications of the polychætous Annelids” (Nature, December 10, 1874); and in this article it is attempted to show what are the simplest forms presenting common features with the Arthropods. The hexapod Insect has been compared by Haeckel, F. Müller, and others to the Zoëæ of Malacostracous Crustaceans, a group in one member only of which, Peneus, has a Nauplius stage been detected. The Zoëæ and the Insect possess alike three pairs of limbs for locomotion, and three for ingestion of food. The abdomen in both is without appendages, and the mandibles are without palps. Admitting the resemblances, there is a prior question to be settled in the case of Insects, Myriapods, and Arachnids. Are the temporary embryonal investments of these animals, the cellular and the structureless membranes, to be compared with the blastodermic moultings of Crustaceans, with that membrane whose presence in the Amphipods is accepted by some observers, as the last trace of the Nauplius stage? In the AcaridsClaparède found the inner layer to invest the embryo after the outer gave way, and Metschnikoff recognises the deutovum in Platygaster also. The identification with the Nauplius is strongly denied by the last-named observer; but there is still room for further investigation, since embryologists of high reputation differ so entirely on the matter of fact, irrespective of phylogenetic theories. Should the identification be accepted, the Arthropods would, as a group, argree in having a Nauplius stage, different in detail in each class; the second, or Zoëa stage, would differ still more in each, and the homologies of the parts would thus become obscured in details, the identity of the general plan being clearly recognisable. All the Arthropods agree in having the terminal portion of the intestinal canal derived from the outer, the middle portion from the inner embryonic layer. Lastly, the Arthropods, in common with the Molluscs and Annelids, have their body-cavity, or perivisceral space, formed by the splitting of the mesoblast, or derivative layer formed between the outer and inner layer (epiblast and hypoblast, and ectoderm and endoderm). In Insects, in the higher Arachnids, and Crustaceans, yelk segmentation is partial; in Myriapods, and the lower Arachnids, and Crustaceans, it is total; but as in Insects, for example, the unsegmented yelk undergoes at a later period a division into polygonal masses, the difference, though of value in classification, is not of primary importance. The relation of the branchiate Arthropods, the Crustaceans, to the other three tracheateclasses, has been discussed chiefly from a phylogenetic point of view. The priority of aquatic to terrestrial forms is assumed, and the derivation of the latter from the former is traced in various ways. The somites or metamera of the Arthropod retain, more than the nervous system which is derived from the epiblast, and still more than the alimentary canal and its appendages, the annulose characteristic that each represents a unit; each may, and many do, give rise to appendages originally similar, but afterwards modified for special functions. Hence the same limbs are tactile in Crustaceans, prehensile in Arachnids, ambulatory in some Crustaceans, accessories of mastication in others, locomotive in some, respiratory in others. In Insects the abdomen of the adult is destitute of appendages, but many larvæ are provided with trachealgills, that is, external processes in which air-canals ramify, and in which a large quantity of blood is received within the cavity of the thin-walled dilatable process. These processes are destitute of external apertures, the tracheal system is in them closed. Such structures are found but rarely to co-exist with the open condition of this tracheal system. But Pteornarcys, one of the Orthopterous order, is remarkable for this conjunction, the branchial processes of the adult overhanging the stigmata of the tracheæ. Considerable variety exists in the position of the gills. The larvæ of Perla have three thoracic pairs of gills, and are terminal of the abdomen. In other cases they are confined to the abdomen. Now, it is to be noted that these gill projections are not at first tracheal; in Chloea the tracheæ appear after another moult, and they are then vibratile. Further, they are developed from the upper surface of the body. Now, in the Annelids the limbs are typically double pairs projected from the sides of the body; the parapodia consist of two branches, notopodium and neuropodium, and the gills when present are modifications of the notopodium. In the Crustaceans this bipartite condition is indicated by the exopodial and endopodial divisions of the limbs. In the adult insect this duplicity has disappeared, unless we recognise in the position of the gilltracheæ the equivalent of the branchiferous notopodium. Gegenbaur and Lubbock regard the wings as trachealgills transferred from locomotive organs in water to locomotive functions in air. Gegenbaur thinks that the dropping off of the gills determines the opening of the tracheal system by the stigmata or pores. Further, he assigns to the closed tracheal system a function similar to that of the swim-bladder of fishes, structures primarily useful in flotation, subordinately respiratory in function.

Natural as may seem the assemblage included under Arthropoda, there is no group in which adaptive modifications have introduced so much diversity of anatomical and physiological relations. Metamorphoses, the changes of form which changes of external conditions have promoted, are met with of very various amount. The progress of the embryo from the first appearance of the blastoderm up to sexual maturity of the adult may be direct, without metamorphosis, or may be retarded by changes of form and habit, rendering the young animal capable of sustaining life under very various conditions. In any one of these stages, even in the adult, multiplication may be provided for by a process of budding, the bud from which the new form emerges being in essence undistinguishable from the ovum for whose further development impregnation is necessary. These metamorphoses are probably of late origin in the history of the group, their perpetuation being due to change in their surroundings. Their relations may be “falsified by the struggle for existence,” the details of the developmental history of the family (phylogenesis) may be crowded into a short space in the development of the individual (Ontogenesis). The description of these variations belongs to the particular treatment of the Crustaceans, Myriapods, Arachnids, and Insects.